1. Field of the Invention
The present invention relates to a method of forming a fluorescent screen, in particular to a method of forming a color filter to be formed on the inner surface of the panel of a color cathode-ray tube.
2. Description of the Related Art
In the conventional color cathode-ray tube, there is known a fluorescent screen in which a filter pattern is interposed between the inner surface of the panel of the face plate (face panel) and a fluorescent layer, wherein the filter pattern is constituted by a pigment layer which is capable of transmitting therethrough a light of the same color as the luminescent color of a fluorescent substance. This fluorescent screen is constructed such that a light of green or blue component among the incident external light is enabled to be absorbed by a red pigment layer, a light of green or red component among the incident external light is enabled to be absorbed by a blue pigment layer, and a light of blue or red component among the incident external light is enabled to be absorbed by a green pigment layer, thereby making it possible to improve the image characteristics such as contrast, color purity, etc.
As for the method of forming such a filter pattern, there has been employed a method, as set forth in Jpn. Pat. Appln. KOKAI Publication No. 11-354026, wherein a pigment-dispersed solution is coated on the inner surface of panel to form a pigment layer thereon, after which a photoresist is coated on the pigment layer. Thereafter, the resultant photoresist layer is subjected to a patterning process comprising exposure and development steps. On this occasion, predetermined regions of the filter layer which are required to be left remain for forming a pattern are required to be excellent in adhesivity to the glass panel, and all of the regions other than the aforementioned predetermined regions are required to be excellent in releasability from the glass panel. Further, the pigment layer should be constructed such that it is excellent in transparency and pigment particles are uniformly dispersed all over the layer without being flocculated.
This method of forming a filter pattern described above however is accompanied with a problem, on the occasion of drying a coated film which has been formed through the coating of a pigment dispersion liquid on the inner surface of the glass panel, that a difference in drying speed is generated between a central region and a corner region of the effective display area of the face panel. As a result, a dried region and an undried region are generated in the middle of the drying step, thereby raising a problem that a rib-like thickened portion is generated at the boundary between the dried region and the undried region.
Namely, since the viscosity of the pigment dispersed liquid is generally as low as 2 cp, when the pigment dispersion liquid is coated on the inner surface of the face panel by means of a rotational coating method, differences in film thickness are generated among a central region, an intermediate region and a peripheral region of the face panel, i.e. the film thickness is increased in the mentioned order. At the peripheral region in particular, i.e. at four corner portions of a rectangular panel, a long and slender thickened region is generated along the rotational direction.
When a coated film having such a non-uniform thickness is dried, a difference in drying speed is generated in accordance with the differences in film thickness, so that dried portions in the central and intermediate regions are caused to co-exist with the undried portion in the peripheral region, in particular, corner regions in the middle of the drying step, thereby permitting a rib-like thickened portion to be generated at the boundary between the dried portion and the undried portion.
If there is such a rib-like thickened portion on the panel face, the portion of resist which is formed over this rib-like thickened portion becomes thinner, so that the ability of the resist to retain the pigment film would be deteriorated. As a result, there will be generated a phenomenon of so-called dot flaws where even a portion of a pigment film which is covered by a resist is removed on the occasion of removing a portion of the pigment layer which is not covered by the resist. A color cathode-ray tube having such dot flaws in the color filter thereof is deemed as being defective.
This problem is not confined to the aforementioned pigment film, but will be also encountered in forming a black film to be employed for forming a black matrix.
In the case where a film is excellent in light transmission, the uniformity of the film can be determined from a transmitted light. Namely, since the light transmittance of a film varies depending on the thickness of the film, the uniformity of the transmitted light depends largely on the uniformity of film thickness such as the irregularity of film thickness. In the case where a film is poor in light transmission, the uniformity of the film can be determined from the reflected light. Since the uniformity of the reflected light is hardly affected by the magnitude of film thickness, the non-uniformity of film thickness can be substantially disregarded.
In the case where a fluorescent screen is provided with a light transmissive color filter as mentioned above, the non-uniformity of film thickness gives much influence to the brightness as well as to the uniformity of the external appearance of the display device such as a CRT. Further, if the film thickness varies greatly, it will give rise to the generation of the aforementioned problem of dot flaws. Therefore, it is important to control the uniformity of the film thickness, and the reason for necessitating this control can be attributed to the drying speed of the film. As explained above, the control of this drying speed becomes very important especially in the case where the film to be controlled is a color filter excellent in light transmission.